Lithospheric detachment of India and Tibet inferred from thickening of the mantle transition zone

Accepted Manuscript Title: Lithospheric detachment of India and Tibet inferred from thickening of the mantle transition zone Author: Yaohui Duan Xiaobo Tian Zhen Liu Gaohua Zhu Shitan Nie PII: DOI: Reference:

S0264-3707(16)30017-5 http://dx.doi.org/doi:10.1016/j.jog.2016.02.001 GEOD 1399

To appear in:

Journal of Geodynamics

Received date: Revised date: Accepted date:

16-6-2015 4-2-2016 5-2-2016

Please cite this article as: Duan, Y., Tian, X., Liu, Z., Zhu, G., Nie, S.,Lithospheric detachment of India and Tibet inferred from thickening of the mantle transition zone, Journal of Geodynamics (2016), http://dx.doi.org/10.1016/j.jog.2016.02.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Lithospheric detachment of India and Tibet inferred from thickening

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of the mantle transition zone

3 Yaohui Duan1,2, Xiaobo Tian1,3*, Zhen Liu1,2, Gaohua Zhu1,2, Shitan Nie1,2

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State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of

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Sciences, Beijing 100029, China.

University of Chinese Academy of Sciences, Beijing, 100049, China

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CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China

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Abstract

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To spatially and temporally interpret eruptive volcanic activity and plateau uplift, the dynamic model

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of the Himalayan-Tibetan orogen requires several scenarios in which the deep part of the lithosphere

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is removed. The removed cold, dense material sank deeply and may rest in the mantle transition zone,

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which is considered as the graveyard for descending mantle lithosphere. Beneath the Himalayas and

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southern Tibet, stacking teleseismic P-wave receiver functions reveals thickening of the mantle

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transition zone (MTZ), which is caused by decreasing temperatures. We interpret the MTZ

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thickening beneath southern Tibet as being a result of a remnant of detached thickened Tibet mantle

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lithosphere, whereas the other thickening is most likely caused by a lithospheric slab that detached

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from the Indian plate and is sinking into the MTZ beneath the Himalayas.

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Corresponding author: [email protected]

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1. Introduction The Tibetan plateau, which was created by the Indian-Eurasian collision and continuous

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northward motion of the Indian Plate since 50-65 Ma (e.g.,Molnar and Tapponnier, 1975; Yin and

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Harrison, 2000), is featured with the highest mountains and largest flat plateau on Earth. The large

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convergence between the Indian and Eurasian plates is estimated to spread 1800-2800 km long

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(Johnson, 2002; Molnar et al., 1993; Replumaz et al., 2013). As a result of this convergence, the

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crust has thickened as twice as the thickness of normal continental crust (e.g.,Tian and Zhang, 2013;

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Xu et al., 2014; Zhang et al., 2011). However, the lithosphere under the plateau has not thickened

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(Zhao et al., 2010). Where did the remaining lithosphere go? Although many seismic observations

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(e.g.,Kumar et al., 2006; Li et al., 2008; Liang et al., 2012; Replumaz et al., 2014; Tilmann and Ni,

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2003; Zhang et al., 2013; Zhao et al., 2010) have been performed to study the collision mechanism,

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no coherent image of the lithospheric mantle in Tibet has been convincingly restructured. Recent

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volcanic activity and plateau uplift eruptions observed in the spatial and temporal domains suggest

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removal of the deep part of the lithosphere (Chung et al., 2005; Ding et al., 2003).

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The removed cold, dense material sank deeply and may rest in the mantle transition zone (MTZ),

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which is considered to be the graveyard for descending mantle lithosphere because of a combined

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positive buoyancy from phase transitions at the lower boundary and increased viscous shear

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resistance across the boundary (Billen, 2008). The MTZ, which is bounded by two sharp seismic

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discontinuities at depths of approximately 410- and 660-km, has been reported to be sensitive to

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temperature (Bina and Helffrich, 1994), and its thickness variations may be an indicator of cold

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subducting slabs or hot deep mantle plumes. A detached lithosphere was revealed beneath the

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Bangong-Nujiang suture via observing a high P-wave velocity near the bottom of the MTZ (Chen

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and Tseng, 2007). Several teleseismic P-wave receiver function (RF) studies have been performed to

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image the lateral variations in the MTZ from south to north under the plateau; however, a thickened

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MTZ, the graveyard for descending mantle lithosphere, has not been observed under northern India

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or Tibet (Kind et al., 2002; Kosarev et al., 1999; Ramesh et al., 2005; Yuan et al., 1997). In this

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work, we performed a modified stacking technique to study the high-resolution variations in the

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MTZ thickness thus the geometry of the MTZ will be found.

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2. Data and methods

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Teleseismic P waveforms recorded by several broadband experiments (see Fig. 1) are downloaded

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from the Incorporated Research Institutions for Seismology (IRIS) database. We select events with

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epicentral distances ranging from 30° to 80° (because the

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multiple reflections from shallow discontinuities when the epicentral distance is greater than 80°)

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and with magnitudes of greater than 5.3. All the records are cut using a window of 10 s prior and 100

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s after the P-wave arrival time, respectively. The RFs are calculated in the frequency domain

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(e.g.,Ammon, 1991) with a band-pass filter of 7~30 s. We totally obtain 2061 high signal-to-noise

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ratio RFs for further processing. Based on the RF analyses, we first determine the time difference

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between converted waves from the 660- and 410-km discontinuities ( t ) and the time delay

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between converted waves from the 410-km discontinuity and the P wave arrival time ( t410 ) in the

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RF. We then estimate the MTZ thickness (H) and average shear-wave velocity ( VS ) for the crust and

Pd 410s

value may be affected by the

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upper mantle. By comparing to a global reference model [IASP91 (Kennett and Engdahl, 1991)], we

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deduce

H (the perturbation in H ) and V (the perturbation in VS ) under the plateau

The effects of topography and epicentral distance on the converted phase delay time are

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moveout-corrected using a 6.4 s/° reference slowness according to the IASP91 model (Kennett and

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Engdahl, 1991). Based on the IASP91 model, the time difference between converted waves from the

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660- and 410-km discontinuities ( t ) and between those from the 410-km discontinuity and P-wave

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arrival ( t410 ) are 23.9 and 44.1 s, respectively, for a RF with a 6.4 s/° reference slowness. The

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profiles (Figs. 1 and S1) are divided into segments (with lengths of 1° along the aa’ profile and 0.5°

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along the bb’, cc’ and dd’ profiles), with each segment overlapping half of its length with adjacent

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ones. RFs are distributed among segments according to the piercing points at a depth of 530 km (Fig.

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S1). The resolution is less than the length of each segment when the converted P-to-S wave travels

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laterally approximately 50-100 km into the MTZ. However, most events used in this work were

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located in the West Pacific subduction zone, and the converted ray paths are approximately

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perpendicular to the profiles (aa’, bb’, cc’ and dd’ in Figs. 1 and S1). These converted waves travel a

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long lateral distance perpendicular to the profiles and a short distance along the profiles. Hence, the

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resolution is not significantly degraded by the long lateral distance traveled in the MTZ.

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An example of estimating the

t 410

and t for a segment is shown in Fig. 2. The converted

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phases, Pds near and in the MTZ are limited in the time window 35~80 s in a RF with a reference

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slowness of 6.4 s/°. We select the positive peaks in the time window as the converted phase if their

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amplitudes are greater than the noise level [0.015, as determined by Yang and Zhou (2001)]. The

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amplitude of the 2nd-root stack (Kanasewich et al., 1973) for each converted phase pair, Pd i s and

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( i  j ), is calculated and recorded at the points at which t Pds

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Pd j s

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i, j tdiff  tdiff  t Pd j s  t Pdi s in the RF spectrum (Fig.2C).

 t Pdi s and

As a result of the lateral seismic velocity variation in the crust and upper mantle, the delay

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times t410 are discrepant between RFs, while the time differences t are not sensitive to the

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lateral velocity variations above the 410-km because of the nearly identical paths of

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Pd 660s

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for a segment (±1 s variation in t ) and the crust and upper mantle are laterally heterogeneous in

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velocity (±2 s variation in t410 ), we can stack all RF spectra into a segment spectrum, in which

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each value is obtained by summing all RF spectra in the range of

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all, we divide the RF-spectrum (Fig. 2C) into some uniform grids with small interval. If a grid

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doesn’t contain any spectra value (The amplitude of the 2nd-root stack for each converted phase

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pair, Pd i s and

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are distributed to the nodes with different weights which are calculated according to the distances.

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Then each new node value is obtained by summing its’ adjacent nodes value ranging from

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and t diff ±1 s. Via this procedure, we can obtain t410 and t from the maximum value using

and

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Pd 410s

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over that depth interval. If we suppose that the MTZ thickness does not vary significantly

t diff ±1 s. First of

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t Pds ±2 s and

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Pd j s ( i  j ) ) then the four nodes of the grid are zero, whereas the spectra value

t Pds ±2 s

and t diff windows from 40 to 50 s and 22 to 28 s, respectively.

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In order to test the improvement of this new stack technique, we build a simple model (Table. 1)

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to synthesis the theoretical RF. Here we set the velocity above 410-km varies from -10% to 5% with

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1% interval to generate sixteen models. Then we calculate sixteen RFs with the same ray parameter

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(Van der Voo et al., 1999). The results of these two stack methods are shown in Fig. S6. The average

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t410

is ~45.48 s and all t are approximately ~23.34 s of these sixteen receiver functions. The

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and t using the traditional linear stack is 47 s and 20.375 s whereas using the new one is

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t410

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45.8 s and 23.2 s. We can get the conclusion that the result of this new method is more close to the

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real model. After estimating

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can collect the t410 and t in a narrow window for each RF more reliably than those picking

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without reference by segment-spectrum. The RFs for every segment in the dd’ profile are shown in

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Fig. S7-9. We then calculate the average t in each segment, which differ slightly from those of

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the segment spectrum. The average t is compared with the IASP91 model to estimate the lateral

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variations in the MTZ thickness. The RFs are again distributed among the segments according to the

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piercing points at a depth of 220 km, and the average t410 value is compared with the IASP91

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model to estimate the lateral seismic velocity variations above 410-km.

and t for each segment in all of the profiles (Fig. S2-5), we

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3. Results

MTZ thickening is identified under the Himalayas and central Tibet (Fig. 3). The bb’ and cc’

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profiles show a normal or slightly thinning of the MTZ under northern India. Under Himalaya

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between the Main Boundary thrust and the Yarlung-Zangbo suture, a thickening of the MTZ is

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visible with a maximum

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(In the IASP91 reference model, the thickness of MTZ is 250 km and t is 23.9 s. We suppose

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that the velocity was constant. So when t is 26 s, the thickness of MTZ is about 230 km then the

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H is 20 km). MTZ thickening also exists beneath central Tibet from the central Lhasa terrane (LST)

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to the central Qiangtang terrane (QTT), with a maximum

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aa’ and dd’ profiles. North of the Jinsha River suture, the aa’ profile exhibits a normal or slightly

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H of 20, 8 and 10 km along the aa’, bb’ and cc’ profiles, respectively.

H of approximately 10 km for both the

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thinned MTZ under Tibet. The large thickness variations (~25-30 km) imply that the south-north

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variations in the temperature of the MTZ are greater than 250℃ (Bina and Helffrich, 1994).

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VS

When estimating the value of

above the 410-km discontinuity, we first consider the

variations in t410 caused by the 410-km uplift (depression) discontinuity. To do this we can use

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t do determine H and get the depth of the 410-km. Specifically there are two conditions, one

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is the Himalayas and south of the JRS in which

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410-km discontinuity and local depression (uplift) of the 660-km discontinuity, the other is from

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central LST to central QTT in which

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and Tseng, 2007), and a normal 410-km discontinuity is suggested. A cold (hot) anomaly in the MTZ

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will cause local uplift (depression) of the 410-km discontinuity and local depression (uplift) of the

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660-km discontinuity with an 8:5 ratio (Bina and Helffrich, 1994). Thus we have determined the

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depth of the 410-km and we can estimate the approximate value of t410 . If this value is close to the

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measurement. Then we can suppose that the

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contribution to t410 is mainly caused by the depth of the 410-km whereas it is affected by both

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VS

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is known. Accordingly, the decreased t410 in the Himalayas is primarily due to uplift of the

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410-km discontinuity.

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H is caused by local uplift (depression) of the

VS

above 410-km is nearly normal and the

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H is owed to the 660-km discontinuity depression (Chen

and depth. Then we can obtain

VS

using the result of t410 because the depth of the 410-km

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Our results indicate a south-to-north decrease in

VS

VS

for the crust and upper mantle. South of

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30°N,

does not obviously differ from the IASP91 model for the aa’, bb’ and cc’ profiles. North

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of 30°N, the aa’ profile indicates that

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increases slightly north of KF. Similar to the aa’ profile, the dd’ profile also exhibits a low

VS

decreases northward to the Kunlun fault (KF) and

VS ( V

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VS ( V

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is approximately -2%) from central LST to central QTT. The lowest

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-4%) is located in the Songpan-Ganzi terrane (SGT). Our results are consistent with the observations

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of a low seismic velocity and hot upper mantle observed in northern Tibet (Kind et al., 2002;

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McNamara et al., 1997; Tilmann and Ni, 2003). The low-to-normal

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discontinuity implies that there is no cold anomaly in the upper mantle under these profiles.

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4. Discussion and conclusions

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values above the 410-km

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VS

is approximately

Temperature variations can cause the thickening or thinning of the MTZ because of the positive

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and negative Clapeyron slopes at the 410- and 660-km discontinuities, respectively (Bina and

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Helffrich, 1994). The MTZ thickening under the Himalayas (where the MTZ is approximately 20 km

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thicker than in the IASP91 model) implies the presence of a subducting slab or a detached

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lithosphere between the 410- and 660-km discontinuities, which reduces the temperature. High

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seismic velocity anomalies have been imaged in the MTZ under the central Himalayas via seismic

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tomography (Van der Voo et al., 1999) which are consistent with the locations of the thickened MTZ.

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The thickness of the MTZ is underestimate due to the high seismic velocity. But the difference is

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small because RF is not sensitive to P-wave velocity. A cold, high-seismic-velocity subducting slab

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in the upper mantle beneath the Himalayas is not favored because of the normal

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the 410-km discontinuity and the absence of deep earthquakes. The Indian lithosphere has been

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underthrusting beneath Tibet sub-horizontally at least to the Bangong-Nujiang suture (BNS)(Chung

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et al., 2005; Owens and Zandt, 1997). The MTZ thickening under the Himalayas is located

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approximately 450 km south of the northern Indian lithosphere. It is likely that a slab of the Indian

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VS

value above

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lithosphere is no longer attached to the Indian plate and has sunk into the MTZ. Given the 40-50

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mm/yr northward migration of the Indian plate (Chung et al., 2005; Wang et al., 2001; Zhang et al.,

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2004), this break-off was completed by about 11-9 Ma. The age of this break-off can also be

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calculated by dividing the distance between the high velocity zone in the MTZ which corresponds to

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the thickened MTZ (Van der Voo et al., 1999) and the northern end of Indian lower crust by the

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convergence rate between India and Eurasia(DeCelles et al., 2002). It is slightly earlier than the time

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that rifts in the southern Tibet began to tension drastically (Lee et al., 2011; Sundell et al., 2013) as a

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result of crust uplift fast due to remove the high density slab.

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A thickening (about 10 km thicker than the IASP91 model) of the MTZ also appears under

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central Tibet from 30° to 32.5°N along the aa’ profile (Fig. 3A) and from 30° to 34°N along the dd’

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profile (Fig. 3C). According to a triplicate waveform study (Chen and Tseng, 2007), we infer that the

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MTZ thickening under central Tibet is mainly caused by the deeper 660-km discontinuity, which is

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most likely a remnant of cold, detached mantle lithosphere that recently sank. The hot upper mantle

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beneath central and northern Tibet provides a favored condition for the removal of thickened

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lithospheric mantle. The episodes of major magmatic activity that began approximately 15 Ma in the

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QTT (Chung et al., 2005) are most likely attributed to this detachment of lithosphere. According to

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Stoke’s law, a sphere’s terminal sinking speed is proportional to the square of the radius of the sphere

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(Schubert et al., 2001); hence, larger spheres sink faster. There is no evidence that validates the

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detachment and sinking of a large dollop or several small dollops. To vertically travel the

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approximately 450 km from the uppermost mantle to the bottom of the MTZ in 15 Ma, the detached

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lithosphere must have sunk an average of 30 mm/yr (Chen and Tseng, 2007). The differential ground

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speed between the BNS and stable Eurasia is approximately 18-22 mm/yr (Wang et al., 2001; Zhang

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et al., 2004). Accordingly, the BNS would have been located approximately ~300 km south of its

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current position 15 Ma ago. If the detached lithospheric mantle plummeted vertically to the MTZ, it

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was approximately located in the uppermost mantle under the low-Pn velocity zone (where there is

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also inefficient Sn propagation) 15 Ma ago (McNamara et al., 1997). We suggest that the mantle

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lithosphere detached from the QTT and some southern part of the SGT at 15 Ma, and the low Pn

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velocity zone began to form synchronously.

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Since the Miocene, two important lithospheric mantle detachment events occurred beneath the

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plateau (Fig. 4). After the Neotethy an slab break-off at a latitude of 20°N during the late

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Eocene-Oligocene (DeCelles et al., 2002; Ding et al., 2003; Van der Voo et al., 1999), the low-angle

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underthrust Indian lithospheric mantle (which possibly included some part of the lower crust) pushed

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northward and directly compressed and thickened the mantle lithosphere of Tibet. We speculate that

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the tip of the slab began rolling back southward from the BNS ~20 Ma ago. The younger thickened

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Tibet mantle lithosphere, densified owing to long-term melt extraction, is susceptible to gravitational

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foundering, and was detached beneath the QTT and the southern part of the SGT at about 15 Ma

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(Chen and Tseng, 2007; Chung et al., 2005). After the southward rollback, the steep Indian slab

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detached from the Indian plate south of the BNS (at a latitude of 28-29°N) during the Middle to Late

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Miocene (approximately 10 Ma ago)(DeCelles et al., 2002; Van der Voo et al., 1999).

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Acknowledgements We cherish the memory of Prof. Zhongjie Zhang, who was still the leader of our research group

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in spirit. We are grateful to the IRIS Data Center for providing waveform data for this study. This

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research is supported by the Strategic Priority Research Program (B) of the Chinese Academy of

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Sciences (Grant XDB03010700) and the National Natural Science Foundation of China (Grants

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41274066). Figures are made using the Generic Mapping Tools (GMT) software package (Wessel

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and Smith, 1998).

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Wang, Q., Zhang, P.-Z., Freymueller, J.T., Bilham, R., Larson, K.M., Lai, X.a., You, X., Niu, Z., Wu,

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J., Li, Y., 2001. Present-day crustal deformation in China constrained by global positioning

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system measurements. Science 294, 574-577.

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Xu, T., Wu, Z., Zhang, Z., Tian, X., Deng, Y., Wu, C., Teng, J., 2014. Crustal structure across the

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Kunlun fault from passive source seismic profiling in East Tibet. Tectonophysics 627, 98-107.

Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan-Tibetan orogen. Annual Review of Earth and Planetary Sciences 28, 211-280.

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Yuan, X., Ni, J., Kind, R., Mechie, J., Sandvol, E., 1997. Lithospheric and upper mantle structure of

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southern Tibet from a seismological passive source experiment. Journal of Geophysical

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Research: Solid Earth (1978–2012) 102, 27491-27500.

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J., 2004. Continuous deformation of the Tibetan Plateau from global positioning system data.

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J., Teng, J.W., 2013. Normal faulting from simple shear rifting in South Tibet, using evidence

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The boundary between the Indian and Asian tectonic plates below Tibet. Proceedings of the

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National Academy of Sciences 107, 11229-11233.

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304 Figure captions:

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Fig. 1. Station locations on a topographic map of Tibet. The stations for different experiments are

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marked with different symbols. The red, blue, and green solid lines indicate the aa’, bb’ and cc’,

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and dd’ profiles, respectively. The gray dashed outline indicates the approximate location of the

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zone within efficient Sn propagation and low Pn velocities (McNamara et al., 1997). Black solid

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lines represent important structural boundaries: the MBT (Main Boundary thrust), YZS

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(Yarlung-Zangbo suture), BNS (Bangong-Nujiang suture), JRS (Jinsha River suture), KF

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(Kunlun fault) and ATF (Altyn Tagh fault).

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Fig. 2. An example estimate of t410 and △t in a segment. The RF (A) is corrected with a 6.4 s/°

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reference slowness, and the converted phases from 410- and 660-km are included in the 35-80 s

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time window (B). The RF-spectrum (C) shows the stacking amplitude for each converted phase

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pair. All RF-spectra in the segment are stacked in (D) to forma segment spectrum. The inset in

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the top-right corner of (D) shows the segment spectrum for the narrow

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to 50 s, with t diff ranging from 22 to 28 s. The maxima of the spectra are at t410 = 44.7 s and

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t = 24.95 s in the segment. The red and blue colors denote the large and small spectra,

t Pds

window from 40

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respectively.

322 Fig. 3. The perturbation ( V ) of the average S-wave velocity in the crust and upper mantle above

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the 410-km discontinuity and the perturbation ( H ) in the MTZ thickness along the four

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profiles (locations are shown in Fig. 1) estimated by the RFs. The error bars are calculated

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using a bootstrap method (Efron and Tibshirani,1986) to determine the 95% confidence

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intervals. Based on the linear correlation,

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double coordinates. t410 is affected by the depth of the 410-km discontinuity and V ; hence,

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t410

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shows the experimental results for PASSCAL 91/92, INDEPTH 2 and BHUTAN 02/03. The bb’

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and cc’ profiles (B) show the HIMNT 01/02 experimental results. The dd’ profile (C) shows the

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INDEPTH 3 experimental results. The location of the low-Pn-velocity zone (McNamara et al.,

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1997) is indicated in (A) and (C).

and t are shown on the same curve using

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H

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is presented with the V using two curves and double coordinates. The aa’ profile (A)

Fig. 4. Stages (since 15 Ma) of the Tibetan plateau evolution illustrated using across section along

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the aa’ profile,as shown in Fig. 1, but not including the complex crustal structure. The positions

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of these structural boundaries at 15 and 10 Ma are reconstructed based on current global

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positioning system velocities in and around the Tibetan plateau with respect to stable Eurasia

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(Wang et al., 2001; Zhang et al., 2004). The volcanic distribution is inferred from a magmatism

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study (Chung et al., 2005). The Indian lithosphere slab rollback at approximately 15 Ma is

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speculative, but it would explain the thickened Tibet lithospheric mantle before 15 Ma and

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propagation and low Pn velocities beneath northern Tibet and abrupt thickening lithospheric

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mantle beneath the north of KF, which is identified in previous seismological

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studies(McNamara et al., 1997; Owens and Zandt, 1997; Kind et al., 2002; Tilmann et al., 2003).

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The current MTZ thickening is vertically exaggerated. The observed abnormity of the MTZ

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thinning somewhat north of the KF has not been supported by other studies.

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steep Indian slab detachment from the Indian plate at 10 Ma. The existence of a steeply dipping Indian lithospheric mantle at 0 Ma beneath the BNS explains the zone within efficient Sn

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Table.1. Simple model to calculate receiver function

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 We used a modified receiver function stacking technique.

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 The mantle transition zone(MTZ) is thickened beneath southern Tibet and Himalaya.

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 The southern Tibet may be the result of a detached thickened Tibet mantle

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lithosphere.

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 Another is most likely caused by a lithospheric slab detached from the Indian plate.

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Table 1 Model to calculate receiver function Thickness (km)

Vp (km/s)

Vs (km/s)

Rho (gm/cc)

1 2 3 4

60 350 250 0

6.3000 8.6650 9.8600 10.3500

3.6000 4.7000 5.4000 5.7500

2.7860 3.5428 3.9252 4.0820

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